Wavelength division multiplexing (WDM) devices are becoming increasing important in fiber-optics sensing system and optical communication systems to enhance transmission capacity and application flexibility. The use of WDM technologies not only provides high speed optical communication links, but also provides advantages such as higher data rates, format transparency, and self-routing (S. Wagner, H. Kobrinski, xe2x80x9cWDM applications in broadband telecommunication networks,xe2x80x9d IEEE Communications Magazine, March, P.22, 1989. L. S. Lome, J. Morookian, S. Monacos, and L. A. Bergman, xe2x80x9cWDM requirements for high performance testbeds,xe2x80x9d SPIE, Vol.2690, P.2, 1996) Over the past twenty years, many kinds of WDM device technologies have been developed and demonstrated. WDM device using dispersion photopolymers, arrayed-waveguide gratings (AWG""s) and dichromated gelatin (DCG) volume holographic gratings have been recently reported. For example see the publications:
[1] G. Georgiou, and A. Boucouvalas, xe2x80x9cHigh-isolation single-mode wavelength-division multiplexer/demultiplexer,xe2x80x9d Electronics Letters, Vol.22, No.2, P.62, 1986.
[2] E. Acosta and K. Iga, xe2x80x9cDesign of a wavelength multiplexer-demultiplexer by the use of planar microlenses,xe2x80x9d Applied Optics, Vol.19, No.16, P.3415,1994.
[3] K. Okamoto, K. Syuto, H. Takahashi and Y. Ohmori,xe2x80x9cFabrication of 128-channel arrayed waveguided grating multiplexer with 25 GHz channel spacing,xe2x80x9d Electronics Letters, Vol.32, No.16, P.1474, 1996.
[4] E. J. Lerner, xe2x80x9cMultiple wavelength exploit fiber capacity,xe2x80x9d Laser Focus World, Vol.33, No.7, P.119, 1997.
[5] Y. Huang, D. Su and Y. Tsai, xe2x80x9cWavelength-division-multiplexing and demultiplexing by using a substrate-mode grating pair,xe2x80x9d Optics Letters, Vol.17, No.22, P.1629, 1992.
[(6] M. R. Wang, G. J. Sonek, R. T. Chen and T. Jannson, xe2x80x9cLarge fanout optical interconnects using thick holographic gratings and substrate wave propagation,xe2x80x9d Applied Optics, Vol.31, No.2, P.236, 1992.
[7] M. M. Li and R. T. Chen, xe2x80x9cTwo-channel surface-normal wavelength division demultiplexer using substrate guided waves in conjunction with multiplexed waveguide holograms,xe2x80x9d Applied Physics Letters, Vol.66, No.3, P.262, 1995.
[8] M. M. Li and R. T. Chen, xe2x80x9cFive-channel surface-normal wavelength division demultiplexer using substrate guided waves in conjunction with a polymer-based littrow hologram.xe2x80x9d Optics Letters, Vol.20, No.7, P.797, 1995.
It would be very advantageous to provide a WDM device exhibiting high angle of incidence on the grating necessary to satisfy the Bragg condition for 1550 nm center wavelength and 1852 lines/mm grating frequency, high element dispersion at the high wavelength end of its spectral bandwidth and polarization independent response.
It is an object of present invention to provide a wavelength division multiplexing device of simple construction with dual-functionality of wavelength selection and beam splitting. It is also an object of the present invention to provide a wavelength division multiplexing device that is polarization independent.
The present invention provides a wavelength division multiplexing device comprising two prisms having a pair of coated, highly reflective opposed faces forming an etalon. A volume phase grating is sandwiched between the two prisms. The volume phase grating is positioned between the highly reflective opposed face portions so that a light beam incident on the etalon undergoes a preselected number of multiple reflections between the highly reflective face portions and a preselected number of traversals through the volume diffraction grating, whereby wavelengths satisfying a Bragg condition on each traversal are diffracted out of the volume grating means.
In this aspect of the invention the two prisms may have the same parameters, and the input side of the first prism and opposed side of second prism have highly reflective surfaces to form a etalon, the volume grating sandwiched between the two prisms.
In another aspect of the invention the prisms may be may be formed of infrared transmitting material such as ZnSe for 1550 nm center wavelength or BK-7 material for visible light.
In another aspect of the invention the volume grating may be a conventional single Bragg grating, multiple superimposed Bragg gratings or binary Bragg supergratings. Because the grating is sandwiched between the two prisms, a protective cover glass is not required to prevent water vapor from affecting the sensitive film and to protect the grating from contaminants.
The two prisms may have two sides, one side with a highly reflective surface for internal reflection and an opposed side for output of the diffraction beam.
In a further aspect of the invention there is provided a method of wavelength division multiplexing. The method comprises providing a volume diffraction means between an input face and an opposed reflecting face to multiply reflect a beam of light through the volume grating means for a preselected number of traversals, with each traversal of the beam through the volume grating means being in a different preselected direction whereby light satisfying a Bragg condition during the traversals is diffracted in a direction different from light diffracted during other traversals through the volume grating means. The method includes detecting light diffracted in the different direction for each traversal of the light beam through the volume grating means. In this aspect of invention two faces are formed by refractive prisms. With them, extraordinary polarized and ordinary polarized light can be separated and combined.
In another aspect of the invention there is provided a method of wavelength division multiplexing. The method comprises providing a volume diffraction means between two reflective faces of two prisms disposed at a preselected angle such that the reflective faces are parallel with respect to one another. The two reflective faces reflect a beam of light through the volume diffraction grating means for a preselected number of traversals, with each traversal of the beam through the volume diffraction grating means being in a different preselected direction, whereby light satisfying a Bragg condition during the traversals is diffracted in a direction different from light diffraction during other traversals through the volume diffraction grating means. The method includes detecting light diffraction in different direction for each traversal of the light beam through the volume diffraction grating means.